The present invention relates to a composite roll for manufacturing metallic strips for use in manufacturing heat transfer tubes internally formed with multiple types of grooves, and the method of manufacturing the composite roll.
For heat exchangers of refrigerators, air conditioners, and so on, heat transfer tubes formed internally with a number of grooves are used to obtain heat transfer efficiency. Since an apparatus having high performance and compact size has recently been desired, there has been an attempt to improve the heat transfer tubes with internal grooves. As the techniques thereof, examined has been the application of heat transfer tubes formed with internal grooves, including a number of single shape grooves and multiple types of grooves in which at least one of cross sectional shapes, lead angles, and sizes is different.
A method of manufacturing the heat transfer tube with grooves includes an art described in Japanese Patent Application Publication Laid-open No. Hei 4-15819. In this method, three or more rolls having protrusions in which at least one of cross sectional shapes, lead angles relative to a rotation direction and sizes is different, are coaxially combined as a roll for use. By sandwiching a smooth metallic strip between the combined roll and a smooth roll, grooves are formed on a surface of the metallic strip. Subsequently, the metallic strip is wound in a tube shape with the processed surface as an inner surface, and is butt welded, to a heat transfer tube with grooves.
The roll in use herein is fixed by tightening the three or more rolls in a removable manner. According to this art, multiple types of grooves may be simultaneously formed on metallic strips, so that the productivity of heat transfer tubes becomes preferable and advanced heat transfer tubes can be manufactured.
However, in repeatedly processing metallic strips by the combined roll, metallic strips with grooves may have preferable quality at the beginning of processing, but groove shapes sometimes deteriorate as the processing quantity increases. Particularly, surfaces that are processed with grooves around a boundary between different rolls in contact with each other sometimes have isolated or deformed grooves due to insufficient processing. In other words, the composite roll for use in the conventional method has poor durability and needs to be exchanged at high frequency, so that the roll is not suitable for mass production and the increase in production costs is a concern.
The present invention was made under the above-noted background. The object thereof is to provide a composite roll which has excellent durability and can process over a long period with stability, in composite rolls for forming multiple types of grooves on metallic strips. It is also an object to provide a method of manufacturing the composite roll of preferable durability with certainty.
When the present inventors examined the surface of the conventional combined roll during use, they found that a processing material enters extremely minute gaps at contacting parts of each roll in the conventional roll due to repeated use. At the same time, it was found that the entered processing material widens the gaps due to contact between the roll and metallic strips, resulting to the deformation or fracture of protrusions around the contacting parts. According to the results, the present inventors have reached the conclusion that joining faces should have no gaps so as to improve the durability of a composite roll, and have come up with the present invention.
Specifically, in a composite roll for manufacturing heat transfer tubes which has two or more grooving rolls having protrusions where at least one of cross sectional shapes, lead angles relative to a rotation direction and sizes is different, by coaxially combining the rolls to form multiple types of grooves on a surface of metallic strips by pressing against the metallic strips, the composite roll for manufacturing heat transfer tubes of the present invention is characterized in that the two or more grooving rolls are joined in one body in a mutually surface contacting state.
To xe2x80x9cjoinxe2x80x9d in the description above herein indicates a joined state by a chemical or material scientific joining method such as welding and brazing. It is a state in which a material is physically combined in one body without discontinuous interfaces such as gaps. This state is distinguished from the state of the conventional combined roll which is joined in one body by tightening, in other words, mechanical joining.
The composite roll relating to the present invention has no gaps to which a processing material enters, and is physically in one body, so that protrusions are not deformed during use. Accordingly, the composite roll has excellent durability, and can process metallic strips continuously over a long period.
Moreover, in joining the grooving rolls to each other, they may be joined by brazing as described above. However, joining strength by brazing is low even though an appropriate brazing material is selected in consideration of a roll material. Thus, it is preferable that the two or more grooving rolls are joined by diffusion bonding. The diffusion bonding joins a material by atomic diffusion between contacting faces. Since an intermediate material such as a brazing material in brazing is not used in the diffusion bonding, joining parts are uniform in a material microstructure. Additionally, joining strength is more preferable than the strength from brazing. Moreover, the roll material is joined without being molten as in welding, so that the grooving rolls may be joined in one body without deforming the protrusions of the rolls before joining.
As a material for a roll continuously pressing metallic strips as in the present invention, it is preferable to use tungsten carbide-based cemented carbide having hardness of 81 to 90 in the Rockwell A hardness (referred to as xe2x80x9cHRAxe2x80x9d, hereafter). General tool steel is also applicable, but cemented carbide is hard and can maintain the durability of rolls. The tungsten carbide-based cemented carbide herein is an alloy in which tungsten carbide (WC) powder is sintered with iron, cobalt or nickel as a binder. The tungsten carbide-based cemented carbide also includes an alloy to which carbides are added such as titanium carbide (TiC), tantalum carbide (TaC), molybdenum carbide (Mo2C), vanadium carbide (VC) and chromium carbide (Cr3C2), besides tungsten carbide. The use of a cemented carbide having hardness of 81 to 90 in HRA is considered preferable because a cemented carbide having less than 81 hardness has insufficient wear resistance. On the other hand, a cemented carbide having more than 90 hardness has sufficient wear resistance, but has less toughness so that the protrusions are likely to be fractured as a roll.
Furthermore, although the tungsten carbide-based cemented carbide in which tungsten carbide is sintered with cobalt as a binder is generally used, it is preferable to apply tungsten carbide-nickel cemented carbide in which nickel is used as a binder in consideration of corrosion resistance. In processing metallic strips, lubricant or processing liquid is sometimes poured between the roll and a processing material in order to prevent the roll and the processing material from sticking and to improve productivity. However, the processing liquid is corrosive. Additionally, in case of applying the tungsten carbide-nickel cemented carbide, it is also preferable to use an alloy having hardness of 81 to 90 in HRA for the same reasons as mentioned above.
Subsequently, a method of manufacturing the roll relating to the present invention will be explained. As previously described, in the present invention, two or more grooving rolls are integrally joined by diffusion bonding. As the method thereof, after the two or more grooving rolls having protrusions where at least one of cross sectional shapes, lead angles relative to a rotation direction and sizes is different, are coaxially combined, the combined grooving rolls are joined by pressing and heating simultaneously.
As the condition thereof, it is preferable to join the grooving rolls by pressing at the heating temperature of lower than a melting point of a binder of the cemented carbide as a component of the rolls, and with the pressure of 1.0 to 5.0 MPa for two to seven hours under a non-oxidizing atmosphere. The heating temperature is limited since the cemented carbide is partially softened or melted when the temperature is at the same or higher than the melting point of a binder of the cemented carbide as a component of the rolls. Accordingly, the grooving rolls are deformed in a joining step, and a composite roll of preferable precision cannot be manufactured. Moreover, the heat holding time is two to seven hours so as to join the rolls with certainty even at relatively low temperature by continuously pressing them over a long period. Additionally, the rolls are joined under a non-oxidizing atmosphere so as to prevent the oxidation of contacting faces of the grooving rolls and to accelerate joining.
Furthermore, in performing diffusion bonding under such conditions, it is preferable to grind joining faces before combining the grooving rolls. Since actual joining faces have oxide film that prevents atomic diffusion, the film has to be removed. Additionally, by flattening the joining faces, an actual contacting area is increased so that joining may be more smoothly performed. It is also preferable that the joining faces have the flatness of less than 5 xcexcm in grinding the joining faces.